Jacob/Israel is dying. Joseph goes to visit, bringing along his two sons. Jacob tells Joseph of the promise God made to him of land and descendants. Jacob then asks for Joseph to bring him his two sons, Ephraim and Manasseh, so that he may bless them and accept them into his lineage. With his blessing, Jacob made it known that the younger boy, Ephraim, would become greater than his older brother.
I find it interesting that Ephraim and Manasseh were able to hear which of the two of them was expected to do better. I see this in a way as setting them up for self-fulfilling prophecy. From now on everyone is going to expect Ephraim to succeed over his brother. This could in turn lead to more opportunities for success, as well as less emphasis placed on Manasseh. This could also be a very early distillation of The Pygmalion Effect. This describes the results from studies that show when teachers expect students to perform better, they do. It was observed that when some students were randomly selected to be labeled as gifted or bright in their student records, they happened to perform better due to teacher expectation.
What all does this matter for the chapter being discussed? Jacob has blessed one child to perform better than the other, in an effort to help fulfill a promise made by God. Everyone now knows that Ephraim will be the one to continue the fulfillment of this. It is very likely that much will be done, even if not consciously, to aid Ephraim in carrying this on, including him passing it on to his progeny if necessary. Even he knows what is expected of him, so he will be driven to behave in ways that will work in favor of completing the promise. All of this, combined with the continual reaffirmation that God promised them land and a multitude of progeny aids in the fulfillment of this promise. The odds are being stacked in their favor.